The Invention relates to a method for determining gasous compounds, wherein a sample gas stream is lead to a sensory array with several gas sensors (2), wherein the electrical output signals of the individual gas sensors (2) are captured by the evaluation computer.
Such methods and apparatuses are employed for the identification of gas mixtures, in particular in the food processing industry.
By now it has become of large importance to investigate and monitor, for example, food products with respect to quality or the environment with respect to a loading of the environment. For this purpose gas mixture samples are taken from the products with desired quality features and characteristic images are generated with the aid of suitable sensors. These images are stored and serve as comparison patterns for all successive investigations.
Such methods are known for a longer time. Apparatuses and arrangements operating according to this method have been employed more heavily in recent times as xe2x80x98electronic nosexe2x80x99. These apparatuses comprise a unit for taking gas samples with a pump and a gas flow sensor, a sensor array with several, for example, 10 different gas sensors, and an evaluation computer. For example, semiconductor gas sensors are unspecific sensors such that in the presence of gas mixtures all sensors react, however, each sensor delivers a different signal. All these different signals result in an overall picture diagram. The shape of this image characterizes the gas mixture and forms the basis as a pattern for the further investigations. It is also known to employ a spectrometer instead of a sensor array with several gas sensors, for example, an ion mobility spectrometer or an optical spectro-scope or a combination of the recited apparatuses.
However, it has been shown that such method is unsuitable for such gas mixtures, where in addition to the compounds of interest there are present also other compounds in much larger concentrations. These are in general easily volatile compounds such as for example ethanol in alcoholic beverages. Because of the high concentration of the ethanol the sensors react mainly relative to the ethanol and not to aroma materials and the sensors thereby falsify the measurement results. A further disadvantage of this method comprises that many gas sensors do not generate linear concentration characterizing curves with the same materials and different concentrations. This leads to different patterns and thus to an increased expenditure for rerecognition of a gas mixture. It is also disadvantageous that the sensors change in the course of time and that for this reason, the patterns have to the renewed again and again.
There exists therefore the purpose to develop a method and an apparatus of the recited kind, which linearizes the characterizing curves of the individual sensors.
A further object comprises to create the possibility to further free, as required, the gas samples in addition from easily volatile gas components.
This object is accomplished the method for determining gaseous compounds, wherein a sample gas stream is thinned with a stream of air prior to the feeding into the sensor array and wherein thereby the concentration of the sample gas stream is kept constant to a predetermined value. The electrical output signal gas sensor array are captured by the evaluation computer and converted into a characterizing diagram, are stored and are compared with the diagrams of other gas mixtures. The apparatus for determining of gaseous compound, comprising also a transfer and control unit (16) for the controlled transport of a sample gas stream through the sensor array (1), a switchable three way valve (6) is disposed in a feed line (3) with an inlet port (4). The switchable three way valve (6) is in connection with a selective collection unit (7) and wherein a sideline (12) with a thinning unit (13) joins between the three-way valve (6) and the sensor array (1).
The recited disadvantages of the state-of-the-art are eliminated with the present invention. Thus, gaseous compounds can now be recognized with high precision and independent of their concentration. This improves the quality of the comparison measurements substantially. The gas sensors are thereby treated with care such that they are loaded only with a concentration degree far below the saturation limit.
The method is also advantageous with respect to that also gaseous compounds with non-selectionable components can be subjected to such a comparative investigation by a preceding treatment of the gas sample stream in a selective collection unit. This extends the field of application substantially for the method.
The apparatus according to the present invention is constructed based on the switchable three way valve of the apparatus dispsoed in the feed line to the sensor array such that the sample gas stream can be lead as desired over the selective collection unit or passed by the selective collection unit. This selective collection unit is equipped in a special way with a reversible feed transport pump, wherein the reversible feed transport pump fills the selective collection unit by its suction power and empties then again the selective collection unit based on its pressure power. This simplifies the device-technical construction of the apparatus.
It is also advantageous to employ generally known absorbents for the enrichment of the selectionable gas components.
The invention is to be illustrated in more detail by way of a schematic illustration.
The device for the performing of the method for determining the gaseous component comprises mainly a sensor array 1 with for example ten gas sensors 2 formed as semiconductor gas sensors. This sensor array 1 comprises on the one hand a feed line 3 with an inlet port 4 and on the other hand a discharge port 5. A switchable three way valve 6 is disposed in the feed line 3, and a selective collection unit 7 is connected to the switchable three way valve 6. This collection unit 7 comprises a special adsorbent 8 and a heater 9 as well as a separate feed transport pump 10 with the a flow sensor 11 for the feed transport pump 10. A sideline 12 connects to the feed line 3 between the three way valve 6 and the sensor array 1, wherein a thinning unit 13 is disposed in the sideline 12. This thinning unit 13 is also furnished with a feed transport pump 14 and a flow sensor 15 for the feed transport pump 14 as well as with an air filter not illustrated. A transport and control unit 16 is disposed in the region of the discharge port 5 of the sensor array 1, wherein the transport and control unit 16 again comprises a feed transport pump 17 and a flow sensor 18 for the feed transport pump 17. An electric conduit branches off from the sensor array 1 and leads to an evaluation computer 20, wherein the evaluation computer 20 is furnished with contact to all flow sensors 11, 15 and 18 through connection lines not illustrated.
A sample is taken off a gas mixture for determining the gas mixture, wherein the feed transport pump 17 of the transport and control unit 16 sucks in a predetermined flow of the gas mixture through the inlet port 4 and transports the predetermined flow of the gas mixture through the sensor array 1. At the same time the feed transport pump 14 of the thinning unit 13 feeds a predetermined flow of filtered fresh air to the sample gas stream in the feed line 3. The mixing ratio of the sample gas stream and of the air stream is selected such that the work point of the gas sensor 2 is fixed in the lower region of its nonlinear concentration curve. The most sensitive or the quickest gas sensor reacts thereby preferably always at a certain work point such that always the same images of different measurements of a same sample gas stream result independently of the concentration of the gas mixture. As required and in the following, the concentration of the gas mixture can be calculated through the mixing ratio displayed at the evaluation computer 20 and therefore known.
The mixing process is continuously monitored and regulated by the evaluation computer 20. The electric signals of all gas sensors 2 are captured at the evaluation computer 20, are optically imaged with a diagram and stored. This diagram characterizing for the measured sample gas stream is then available for later identification of other gas mixtures as a sample pattern.
If a gas mixture is present with a part of easily volatile components, wherein the easily volatile components are not selectionable based on their approximately same concentration, and wherein the easy volatile components, because of their high concentration, mainly act on the gas sensors 2, then the selective collection unit 7 is employed. The feed transport pump 10 of the selective collection unit 7 sucks the sample gas stream into the adsorber 8, wherein the feed transport pump 10 is automatically controlled by the evaluation computer 20 and by the flow sensor 11. The required medium volatile and hardly volatile aroma materials of the gas mixture are here enriched on special adsorbents, while the easily volatile gas components break through and thus are not enriched. A following thermal desorption, introduced by the heater 9, dissolves the medium volatile aroma materials from the absorbents. These aroma materials are lead back again into the feed line 3 based on a computer controlled reversal of the transport direction of the feed transport pump 10 and these aroma materials are transported in and through the sensor array 1 in cooperation with the feed transport pump 17 of the transport and control unit 16. At the same time again filtered fresh air is mixed in through the thinning unit 13 in the manner already described.